Investigators on Friday poured into San Diego, Yuma and Folsom, home of California’s main power system operator, to figure out how a modest failure in the Arizona desert became the worst blackout in San Diego history.

They will spend months determining whether the problem was a mistake by one utility worker, a massive breakdown of a system that is supposed to maintain reliability above all — or both.

According to San Diego Gas & Electric, an Arizona utility worker went to fix faulty equipment at a substation and triggered the meltdown that left almost 7 million people in two countries in the dark for hours.

Some experts already are convinced the system shutdown was avoidable, only because so many failsafes are supposed to be in place.

“Every possible contingency is planned for,” said Robert McCullough, a utility consultant from Portland, Ore. who has been following the San Diego case. “Unless there’s a catastrophic event like a tsunami or an earthquake, we should have been prepared.”

Officials at several government agencies announced investigations, inquiries and special reviews of the failure, which stretched from Orange County into Mexico and persisted in some places for more than 24 hours.

No one would discuss why the system was unable to withstand the Arizona mishap, which started with a transmission line shutdown between Phoenix and Yuma.

“When you have a blackout of this magnitude, there is an extensive investigation,” said Stephanie McCorkle, spokesowman for the California Independent System Operator in Folsom. “In this case, there are several investigations under way. We’ve kicked them off today and we’re cooperating fully with the agencies involved.”

Cal-ISO, the nonprofit that manages the majority of the state power grid, is responsible for three of the five worst blackouts in San Diego County between 1988 and 2010.

In those cases, the system operator ordered power plants and customers shut down to avoid a larger-scale interruption. No such powering down happened as a preventive measure on Thursday.

“We were actually bringing generation back up — not taking it off,” said McCorkle, who declined to say which plants were being jump-started or whether the fresh power was headed to Yuma.

Michael Shames of the Utility Consumers’ Action Network, a critic of the power company, said the issue is likely broader than the incident in Yuma.

“We know that (alone) couldn’t have taken the San Diego system down,” he said. “Something else must have happened.”

Shames, who has sued SDG&E many times over the years, said he plans to seek a subpoena for the operating logs so he and his colleagues can review for themselves what specifically caused the blackout.

Among the many agencies now analyzing the event are the Federal Energy Regulatory Commission, the North American Electric Reliability Corp. and the Western Electricity Coordinating Council.

Rachel Sherrard of the coordinating council said her group opened what’s known as an “event analysis,” which is not an investigation but instead focuses on how to learn from whatever mistakes were made.

“Obviously we’re trying to identify and ensure timely implementation of any corrective action,” she said. “Our focus is on reliability and the focus of the event analysis is ultimately to share lessons learned throughout the industry.”

SDG&E President Mike Niggli described a cascading series of outages after the lines to Arizona shut down, severing one of two vital links between San Diego and the electrical grid for the Western United states.

That triggered the shutdown of the San Onofre Nuclear Generating Station and transmission lines that lead north from San Diego — the area’s second lifeline to the grid.

Cut off from the wider grid, the local demand for electricity outstripped the supply, destabilizing the system. Power plants then shut down to avoid damage, and San Diego County went dark as the sun set.

“Our ties to the outside world were gone and that’s when things resulted in having too much customer demand and not enough power to serve them,” Niggli said. “When you have one event, generally you don’t see this kind of an end-game response. So that’s why all of us are going to investigate: What was the linkage between what occurred in Arizona and the San Onofre lines coming offline? To me, that’s where the focus will be.”

Utility officials in Yuma also were struggling to explain what happened.

The first sign of trouble came at 3:27 p.m. Thursday as a worker was switching out a series capacitor — a voltage regulating device the size of a small car — at a substation in North Gila, outside Yuma.

That’s when power went out along a 110-mile section of high-capacity transmission line stretching from North Gila to Hassayampa outside Phoenix, said Jim McDonald, a spokesman for APS, Arizona’s largest utility.

Customers weren’t affected, however, until 3:38 p.m., he said, adding that it was too early to know whether work on the capacitor triggered the initial outage on APS’s transmission line, and how that led to broader failures stretching into California.

“Typically it would have been contained right there and nobody would have lost service,” McDonald said.

Kelly LaRosa had no interest Friday night in speculation by power companies or pending investigations. She was still lacking electricity and works from home.

“I’ve been sitting in my car charging my work phone, then I go back to my house and make calls to my clients,” said La Rosa, an insurance agent who lives in a 400-unit townhome complex in Linda Vista.

She was most annoyed that Niggli was on television Friday saying SDG&E had fully restored power.

“It was too soon for them to pat themselves on the back,” she said. “I called, but you can’t get through ... They have outgoing messages saying they know they have a problem.”

Merwin Brown is a researcher for the California Institute for Energy and Environment, a think tank at the University of California developing new tools to improve the transmission and distribution of electricity.

Brown said the Cal-ISO is one of the most progressive operators when it comes to investing in emerging technology.

Among his recent projects are “synchrophasors” to give operators real-time monitoring and alerts about power levels across the grid.

“The old system is like an X-ray,” Brown said. “The new system is like an MRI — you have much higher resolution.”

Brown predicted that within five or 10 years, operations across the grid will be upgraded with better technology and more outages will be prevented.

Shames, the utility consumers’ advocate, said even if problems with grid operations are resolved through technological advances, regulators must find out why telecommunications, water and sewage networks were disrupted more than usual.

“These are important questions, almost as important as what caused the electrical outage,” he said. “The stakes are really high now.”